With increasing interest in antilock braking systems, there has been a concurrent interest in wheel speed sensor assemblies. Such assemblies typically incorporate a sensor ring that turns at wheel speed, and a stationary sensor that faces the sensor ring. In the case of wheel bearings that have a large sealed interior volume, it is a simple matter to mount the sensor components in that existing space. This is especially true for non-driven vehicle wheels, which have no axle shaft passing through the center of their bearings, and which can thus be sealed on one side with a cap. The bearing cap provides a large, easily accessed space in which to mount the sensor components. An example of such a design may be seen in FIG. 1 of Kruse U.S. Pat. No. 3,683,219.
In the case of driven wheels, such as those in conventional rear drive vehicles, axle shafts extend in either direction from a differential, through a stationary axle tube and eventually through bearings that support the axle shaft within the axle tube. A long, thin volume is thereby created between the shaft and tube. The rear wheels are mounted to the ends of the axle shafts. Since the axle shaft rotates at vehicle wheel speed, it is a logical place to mount the sensor ring. Likewise, since the axle tube is stationary, it provides a solid foundation for the sensor. However, the thin annular space is not nearly so accessible for the installation of the sensor components as is the end cap found on non-driven wheel bearings. FIG. 2 of the patent noted above shows a sensor assembly in such an annular space, but is notably silent as to how it would be installed.